This invention relates to a freezing chuck type machining method, and particularly to a method for fixing a workpiece in position by freezing a chucking medium.
When carrying out machining such as milling, grinding, turning, polishing, cutting, dicing, boring or engraving on a workpiece it is necessary to firmly fix (chuck) the workpiece to a machine tool table.
Conventionally, a magnetic chuck, a vacuum chuck or a vice has been used for this workpiece fixing, but there are limits on the materials and shapes of workpiece that can be chucked with these chucking devices, and there has been the problem that when the workpiece is thin or fragile or of a complex shape it is not possible to fix the workpiece certainly and stably.
As a solution to this, freezing chuck methods and devices using ice as an adhesive medium have been proposed. In this related art technology, water is coated in spray form onto a freezing plate made of a material having good thermal conductivity such as copper, a workpiece is placed on this and then the temperature of the top face of the freezing plate is reduced to below 0.degree. C. by a current being passed through thermoelements disposed below the freezing plate and the water is thereby frozen and the work is fixed by a film of ice.
However, with this related art technology there have still been the following kinds of problem.
(1) It is difficult or impossible to machine using a machining liquid.
Heat of working produced between the workpiece and the tool during machining can severely damage the machined surfaces and also shorten the life of the tool. To avoid this, also in this method wherein the workpiece is fixed by freezing, it is preferable to use a machining liquid or a coolant liquid (hereinafter both referred to with the term machining liquid) as is done in general machining methods. However, the temperature of the machining liquid generally is higher than the temperature of the ice fixing the workpiece. Consequently, when machining liquid is supplied to the machining point the ice film is melted, and this is extremely dangerous because during machining the workpiece not only detaches from the fixing surface so that machining becomes impossible but also flies off the table.
When to avoid this a water-soluble machining liquid containing an antifreeze is used and this is cooled to below freezing point before use, because ice has a strong affinity for water the machining liquid still tends to melt the ice fixing the workpiece and releasing of the fixing of the workpiece readily occurs. For these reasons, in practice it has only been possible to apply freezing type chuck methods of the related art to dry machining, wherein no machining liquid is used.
(2) Movement of the tool tends to be obstructed by ice laminating onto the workpiece during machining.
For example when cutting or dicing of a workpiece is carried out, water from a water-soluble machining liquid or from the air freezes and forms a layer of ice on the workpiece during machining. Because this ice makes contact with the tool flange or mandrel and obstructs the movement of the tool, accurate machining becomes difficult and trouble such as damage to the tool or the main spindle has tended to occur.
(3) Compared with ordinary chucking means, the workpiece chucking and removal efficiency is poor and also the cost is high.
In the related art technology, water is frozen and the workpiece is fixed by a film of ice, but in practice it has not been possible to obtain a sufficient fixing strength without reducing the temperature of the top surface of the freezing plate to a temperature lower than -5.degree.C., for example -10.degree.C., and it has not been possible to remove the workpiece after machining without raising this temperature to above 0.degree.C. Consequently, a long time of one to three minutes has been required for fixing of the workpiece on the freezing plate and also for removing the workpiece from the freezing plate.
As ways of overcoming this, conventionally the following kinds of method have been adopted. That is, a freezing chuck device additionally provided with a vacuum chuck function and a freezing chuck device not having a vacuum chuck function have been used and the former has been disposed on the table of the machine tool and the latter has been disposed off the machine tool. Several aluminum adaptor plates have been prepared in addition to these two freezing chuck devices, and a cycle wherein while a workpiece being machined is held using the freezing chuck device on the machine the next workpiece is fixed to an adaptor plate has been adopted.
That is, first, a film of antifreeze is formed on the top of the freezing plate of the freezing chuck device off the machine tool, water is coated onto the top of an adaptor plate placed on this and a workpiece is placed on this water, the freezing chuck device is then operated and the workpiece is thereby fixed in advance to the adaptor plate by freezing and is held on standby in this state. Then, when the machining of the workpiece on the machine tool has been finished the vacuum chuck of the freezing chuck device on the machine tool is operated to release an adaptor plate which had been fixed to that freezing chuck device by the vacuum chuck and the adaptor plate standing by as described above is placed on the freezing plate (on the top of which has been laid a film of antifreeze) of the freezing chuck device on the machine tool and fixed to this freezing plate by the vacuum chuck being operated.
However, with this method, two expensive devices, one for on the machine tool and one for off the machine tool, each essentially comprising a freezing plate, a thermoelement for reducing the temperature of the top surface of the freezing plate to a low temperature below -5.degree.C., a plastic body provided with a cooling water passage for removing heat from the thermoelement and a mounting block on which the plastic body is mounted, are necessary. Consequently there has been the problem that the equipment cost doubles and running costs such as the cost of electric power consumed are also high. Furthermore, if the freezing plate of the freezing chuck device is not cooled to a temperature below 0.degree. C. the workpiece slips out of position on the adaptor plate and machining by automatic control becomes difficult.